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Large area isotopic silicon targets for astrophysical reaction rate studies in 26Si
Nuclear Instruments and Methods in Physics Research B 241 (2005) 1006–1008 www.elsevier.com/locate/nimb
Large area isotopic silicon targets for astrophysical reaction rate studies in 26Si John P. Greene a
a,*
, Georg P.A. Berg
b
Physics Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA b Kernfysisch Versneller Intituut, 9747 AA Groningen, The Netherlands Available online 8 September 2005
Abstract For measurements of stellar reaction rates of proton rich nuclei involving resonance levels just above threshold, targets of 28Si were used in studies of the 28Si(4He,6He)26Si reaction using the Research Center for Nuclear Physics (RCNP) Ring Cyclotron at Osaka University. Resonance structure observed in 26Si above the 5.5 MeV proton threshold identified levels populated in the astrophysically important reaction 26Al(p,c)26Si and help to determine the yield of 26 Al c-rays seen in Novae. A few isolated states were also observed above the 9.2 MeV a threshold in the (4He,6He) reaction which can be used to improve the present stellar rate calculations. We describe here the involved preparation of large area 28Si metal targets used for these experiments produced by the method of electron beam evaporation. The parting agent used was betaine. To obtain the 0.7 mg/cm2 28Si target thickness needed for the experiment, several mounted layers were assembled together by stacking. This formidable approach therefore required many foils. Two targets stacks of 0.7 mg/cm2 and 0.5 mg/cm2 28Si were delivered to Osaka for the run. Published by Elsevier B.V. PACS: 26.50.+x; 29.25.Rm; 81.15.Ef Keywords: Silicon targets; Electron beam deposition; Betaine parting agent
1. Introduction and scientific motivation For a more complete understanding of the rapid proton capture (rp-process) and to gain *
Corresponding author. Tel.: +1 630 252 5364; fax: +1 630 252 9647. E-mail address: [email protected] (J.P. Greene). 0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2005.07.216
spectroscopic information on astrophysical rate calculations, studies of proton rich nuclei 22Mg and 26Si measured just above proton breakup threshold are of keen interest. For this reason, experiments using thick, 28Si targets were needed for measurements of the resonance levels in 26Si above the proton and a thresholds. These levels above the proton threshold in 26Al(p,c)26Si can
J.P. Greene, G.P.A. Berg / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 1006–1008
determine the population and destruction of the 26 Al c-source emission in novae. Above the a threshold, these levels are important for testing the Hauser–Feshbach level density assumptions. Previous measurements of the a-beam induced 2-neutron pick up reaction, 24Mg(4He,6He)22Mg at 206 MeV were performed using the Osaka University Cyclotron [1]. The isolated resonance states observed were relevant for the rate calculations of the 21Na(p,c)22Mg proton capture as well as the 18Ne(4He,p)21Na reaction which governs break-out of the hot CNO (carbon–nitrogen– oxygen) cycle under X-ray burst conditions.
2. Target foil preparation The preparation of silicon target foils follows the original work of Hinn [2]. For our purposes, electron beam deposition (see [3]) was employed starting from the metal then onto standard microscope slide substrates. Betaine was applied to the slides as a parting agent [4]. This solution was prepared fresh in a mixture of 7:1 by weight betaine anhydrous 98% (Aldrich Chemical Co., Milwaukee, WI 53233, USA) and deionized water. First attempts at growing foils were done using natSi metal. The source to substrate distance was 14 cm and the film thickness was ascertained using a quartz crystal deposition monitor. As thick, self-standing silicon films can be problematic to prepare, several natSi targets were produced with thickness only in the range of 200 lg/cm2. Internal stress build-up in the growing films prevented thicker foils from being successfully prepared. An added difficulty was the rather large aperture of the Osaka cyclotron target frames demanded that the entire 2.5 cm · 7.5 cm substrate be employed to cover the opening. To reach the desired final target thickness of 1 mg/ cm2 several mounted foils on frames would need to be assembled as a ‘‘stack’’ target. This delicate technique had been developed previously by this laboratory and employed in the making of 13C cyclotron targets [5] for experiments at Michigan State University, National Superconducting Cyclotron Laboratory (NSCL). The stacking and assembling was accomplished using a series of tar-
1007
get frames attached to a ‘‘base’’ target frame using small screws in the four corners. After the technique was mastered, isotopically enriched 28Si of 99.94% as solid metal pieces (ISOTEC USA, San Francisco, CA 94129, USA) was used as starting material in the electron beam source. Precautions were undertaken to obtain the lowest oxygen impurity material available (200–10000 ppm oxygen admixture) in order to avoid producing reactions on oxygen from the beam. The isotopic sample was first consolidated by melting with the evaporation proceeding as before onto betaine treated glass substrates. Fourteen 28Si mounted foils were successfully obtained using the standard procedure of floating in a water bath. A thin film of low vapor pressure vacuum grease (Apiezon L) was applied to the target frame, a technique used to adhere the film to the frame.
3. Results and analysis In the end, four ‘‘stack’’ 28Si targets with thicknesses of 709, 699, 688 lg/cm2 and one additional target of thickness 496 lg/cm2 were prepared for the experiment. Fig. 1 shows a photograph of one of the completed target ‘‘stacks.’’ Measurements were performed on these targets using the 28 Si(4He,6He)26Si reaction at 206 MeV using the Grand Raiden (GR) spectrometer in the West Experimental Hall using the south inlet port (WS beam line) at the Osaka University RCNP ring cyclotron. The standard vertical drift chamber (VDC) focal plane detector system was employed
Fig. 1. Completed 0.7 mg/cm2
28
Si target ‘‘stack’’.
1008
J.P. Greene, G.P.A. Berg / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 1006–1008
the a threshold only a few states are seen. These levels however, are important for improving the rate calculations performed under the current theoretical level density assumptions.
4. Conclusion In conclusion, several large area targets of 0.7 mg/cm2 28Si were prepared and used in measurements of the (4He,6He) astrophysical reaction rates using the Grand Raiden spectrometer at the RCNP ring cyclotron at Osaka University in Japan. Resonance levels in 26Si were observed above the proton and a thresholds which shed new light on the 26Al c-ray emission in novae and to improve level density calculations. Plans are underway to use these targets to study the a threshold in 26Si via the (p,t) reaction at 100 MeV incident proton energy.
Acknowledgements
Fig. 2. Spectra of the 28Si(4He,6He)26Si reaction at 206 MeV (65 keV resolution). Proton and a thresholds are given as dotted lines in the top and bottom panels, respectively.
[6], providing E, DE and timing signals for particle identification. In some instances, Al absorbers for additional energy loss were used to optimize detector signals. The energy resolution was 65 keV and carbon and oxygen impurities have been subtracted. Measured spectra are given in Fig. 2 [7]. Several resonances are observed above the proton and a thresholds of 5.5 and 9.2 MeV, respectively. States above the proton threshold identify resonance levels in 26Al(p,c)26Si which give rise to the 26 Al c-ray emission observed in novae. Above
The authors would like to thank Dr. Donald Geesaman and Dr. Kim Lister, for their continuing encouragement and support of these efforts. This work is supported by the US Department of Energy, Nuclear Physics Division, Contract No. W-31-109-ENG-38.
References [1] G.P.A. Berg, K. Hatanaka, M. Wiescher, H. Schatz, et al., Nucl. Phys. A 718 (2003) 608c. [2] G.M. Hinn, INTDS Workshop Proc., Argonne National Laboratory, ANL/PHY-84-2 (1983) 89. [3] G.E. Thomas et al., Nucl. Instr. and Meth. A 303 (1991) 162. [4] P. Maier-Komor, Nucl. Instr. and Meth. 102 (1972) 485. [5] J.P. Greene et al., Nucl. Instr. and Meth. A 438 (1999) 52. [6] T. Wakasa et al., Nucl. Instr. and Meth. A 482 (2002) 72. [7] G.P.A. Berg et al., KVI Annual Report (2003) 1.
Large area isotopic silicon targets for astrophysical reaction rate studies in 26Si John P. Greene a
a,*
, Georg P.A. Berg
b
Physics Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA b Kernfysisch Versneller Intituut, 9747 AA Groningen, The Netherlands Available online 8 September 2005
Abstract For measurements of stellar reaction rates of proton rich nuclei involving resonance levels just above threshold, targets of 28Si were used in studies of the 28Si(4He,6He)26Si reaction using the Research Center for Nuclear Physics (RCNP) Ring Cyclotron at Osaka University. Resonance structure observed in 26Si above the 5.5 MeV proton threshold identified levels populated in the astrophysically important reaction 26Al(p,c)26Si and help to determine the yield of 26 Al c-rays seen in Novae. A few isolated states were also observed above the 9.2 MeV a threshold in the (4He,6He) reaction which can be used to improve the present stellar rate calculations. We describe here the involved preparation of large area 28Si metal targets used for these experiments produced by the method of electron beam evaporation. The parting agent used was betaine. To obtain the 0.7 mg/cm2 28Si target thickness needed for the experiment, several mounted layers were assembled together by stacking. This formidable approach therefore required many foils. Two targets stacks of 0.7 mg/cm2 and 0.5 mg/cm2 28Si were delivered to Osaka for the run. Published by Elsevier B.V. PACS: 26.50.+x; 29.25.Rm; 81.15.Ef Keywords: Silicon targets; Electron beam deposition; Betaine parting agent
1. Introduction and scientific motivation For a more complete understanding of the rapid proton capture (rp-process) and to gain *
Corresponding author. Tel.: +1 630 252 5364; fax: +1 630 252 9647. E-mail address: [email protected] (J.P. Greene). 0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2005.07.216
spectroscopic information on astrophysical rate calculations, studies of proton rich nuclei 22Mg and 26Si measured just above proton breakup threshold are of keen interest. For this reason, experiments using thick, 28Si targets were needed for measurements of the resonance levels in 26Si above the proton and a thresholds. These levels above the proton threshold in 26Al(p,c)26Si can
J.P. Greene, G.P.A. Berg / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 1006–1008
determine the population and destruction of the 26 Al c-source emission in novae. Above the a threshold, these levels are important for testing the Hauser–Feshbach level density assumptions. Previous measurements of the a-beam induced 2-neutron pick up reaction, 24Mg(4He,6He)22Mg at 206 MeV were performed using the Osaka University Cyclotron [1]. The isolated resonance states observed were relevant for the rate calculations of the 21Na(p,c)22Mg proton capture as well as the 18Ne(4He,p)21Na reaction which governs break-out of the hot CNO (carbon–nitrogen– oxygen) cycle under X-ray burst conditions.
2. Target foil preparation The preparation of silicon target foils follows the original work of Hinn [2]. For our purposes, electron beam deposition (see [3]) was employed starting from the metal then onto standard microscope slide substrates. Betaine was applied to the slides as a parting agent [4]. This solution was prepared fresh in a mixture of 7:1 by weight betaine anhydrous 98% (Aldrich Chemical Co., Milwaukee, WI 53233, USA) and deionized water. First attempts at growing foils were done using natSi metal. The source to substrate distance was 14 cm and the film thickness was ascertained using a quartz crystal deposition monitor. As thick, self-standing silicon films can be problematic to prepare, several natSi targets were produced with thickness only in the range of 200 lg/cm2. Internal stress build-up in the growing films prevented thicker foils from being successfully prepared. An added difficulty was the rather large aperture of the Osaka cyclotron target frames demanded that the entire 2.5 cm · 7.5 cm substrate be employed to cover the opening. To reach the desired final target thickness of 1 mg/ cm2 several mounted foils on frames would need to be assembled as a ‘‘stack’’ target. This delicate technique had been developed previously by this laboratory and employed in the making of 13C cyclotron targets [5] for experiments at Michigan State University, National Superconducting Cyclotron Laboratory (NSCL). The stacking and assembling was accomplished using a series of tar-
1007
get frames attached to a ‘‘base’’ target frame using small screws in the four corners. After the technique was mastered, isotopically enriched 28Si of 99.94% as solid metal pieces (ISOTEC USA, San Francisco, CA 94129, USA) was used as starting material in the electron beam source. Precautions were undertaken to obtain the lowest oxygen impurity material available (200–10000 ppm oxygen admixture) in order to avoid producing reactions on oxygen from the beam. The isotopic sample was first consolidated by melting with the evaporation proceeding as before onto betaine treated glass substrates. Fourteen 28Si mounted foils were successfully obtained using the standard procedure of floating in a water bath. A thin film of low vapor pressure vacuum grease (Apiezon L) was applied to the target frame, a technique used to adhere the film to the frame.
3. Results and analysis In the end, four ‘‘stack’’ 28Si targets with thicknesses of 709, 699, 688 lg/cm2 and one additional target of thickness 496 lg/cm2 were prepared for the experiment. Fig. 1 shows a photograph of one of the completed target ‘‘stacks.’’ Measurements were performed on these targets using the 28 Si(4He,6He)26Si reaction at 206 MeV using the Grand Raiden (GR) spectrometer in the West Experimental Hall using the south inlet port (WS beam line) at the Osaka University RCNP ring cyclotron. The standard vertical drift chamber (VDC) focal plane detector system was employed
Fig. 1. Completed 0.7 mg/cm2
28
Si target ‘‘stack’’.
1008
J.P. Greene, G.P.A. Berg / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 1006–1008
the a threshold only a few states are seen. These levels however, are important for improving the rate calculations performed under the current theoretical level density assumptions.
4. Conclusion In conclusion, several large area targets of 0.7 mg/cm2 28Si were prepared and used in measurements of the (4He,6He) astrophysical reaction rates using the Grand Raiden spectrometer at the RCNP ring cyclotron at Osaka University in Japan. Resonance levels in 26Si were observed above the proton and a thresholds which shed new light on the 26Al c-ray emission in novae and to improve level density calculations. Plans are underway to use these targets to study the a threshold in 26Si via the (p,t) reaction at 100 MeV incident proton energy.
Acknowledgements
Fig. 2. Spectra of the 28Si(4He,6He)26Si reaction at 206 MeV (65 keV resolution). Proton and a thresholds are given as dotted lines in the top and bottom panels, respectively.
[6], providing E, DE and timing signals for particle identification. In some instances, Al absorbers for additional energy loss were used to optimize detector signals. The energy resolution was 65 keV and carbon and oxygen impurities have been subtracted. Measured spectra are given in Fig. 2 [7]. Several resonances are observed above the proton and a thresholds of 5.5 and 9.2 MeV, respectively. States above the proton threshold identify resonance levels in 26Al(p,c)26Si which give rise to the 26 Al c-ray emission observed in novae. Above
The authors would like to thank Dr. Donald Geesaman and Dr. Kim Lister, for their continuing encouragement and support of these efforts. This work is supported by the US Department of Energy, Nuclear Physics Division, Contract No. W-31-109-ENG-38.
References [1] G.P.A. Berg, K. Hatanaka, M. Wiescher, H. Schatz, et al., Nucl. Phys. A 718 (2003) 608c. [2] G.M. Hinn, INTDS Workshop Proc., Argonne National Laboratory, ANL/PHY-84-2 (1983) 89. [3] G.E. Thomas et al., Nucl. Instr. and Meth. A 303 (1991) 162. [4] P. Maier-Komor, Nucl. Instr. and Meth. 102 (1972) 485. [5] J.P. Greene et al., Nucl. Instr. and Meth. A 438 (1999) 52. [6] T. Wakasa et al., Nucl. Instr. and Meth. A 482 (2002) 72. [7] G.P.A. Berg et al., KVI Annual Report (2003) 1.